1. Field of the Invention
The invention relates to a focusing control method for an optical drive, and particularly relates to an optical drive with a lightscribe function for producing accurate laser-scribed patterns on the label side of a disc.
2. Description of the Prior Art
For an optical drive with a lightscribe function, the desired label pattern is scribed by a laser beam emitted from the optical pickup unit and focusing on a label side of the disc that is spread with a layer of coating material. When the emitted power of the laser beam exceeds a threshold, the label side of the disc will change and the label pattern will be scribed. However, the optical pickup unit has to constantly focus the laser beam on the label side of a spinning disc in order to obtain the desired label pattern.
When focusing on the data side of a disc, it is known that the pickup unit is driven by a voltage-generated electromagnetic force to move up and down. The laser beam from the pickup unit is then sent to the disc. As the emitted laser beam is reflected by the data side of the disc, it illuminates a transducer. The focusing error (FE) signal is generated according to the magnitude of different illuminated positions. As the FE signals are input to a focusing serve unit according to the magnitude of the FE signal, the optical pickup unit is moved up and down to keep the laser beam constantly focused on the data side. However, the coating material of the label side is not as uniform as that of the data side and the reflection of the light on the label side is not as good as that of the data side. The magnitude and the stability of the FE signal reflected from the label side are not adequate for the focusing serve unit.
FIG. 1 illustrates a conventional label side focusing control method. The optical drive drives an object lens 2 of the pickup unit 1 to move up and down according to the voltage-generated electromagnetic force and keeps the pickup unit 1 emitting a laser beam on the label side of the disc D. As the laser beam is reflected from the label side of the disc to the pickup unit 1, it illuminates the transducer 3. The transducer 3 comprises four equal-size optical receiving sections A, B, C, and D. Each of the optical receiving sections receives different reflected lights from different zones of the reflected laser beam and converts the different reflected lights into corresponding electronic signals. After the electronic signals corresponding to A, B, C, and D are amplified and summed by the amplifier 4, the resultant signal with greater magnitude, called the light summed signal SS, is generated. When the pickup unit 1 moves up and down by a predetermined distance P, the amplifier 4 generates the relevant light summed signal SS with a waveform WS. When the focal point FP is exactly located on the label side of the disc, the magnitude of the light summed signal SS is the maximum. The magnitude of the reflecting light may decrease when the focal point FP moves away from the label side, however. If the label side is placed at the middle of the predetermined distance P, the waveform WS of the light summed signal SS is symmetric with respect to the middle of the predetermined distance P. The sum of the waveform WS in the upper half of the predetermined distance P is equal to the sum of the waveform WS in the lower half of the predetermined distance P (the waveform in the solid line). Otherwise, the sum of the waveform WS in one portion may be larger than the sum of the waveform WS in another portion when the label side is placed at the corresponding portion with respect to the middle of the predetermined distance P (the waveform in the dashed line).
Currently, in the general focusing control method for the label side, the difference between the light summed signals of the top and bottom side of the label side is used to determine the focal point and offset of the label side and thus control the focusing. FIG. 2 shows corresponding curves of the voltage signals for determining the difference of the light summed signals. The label side of a disc is sectioned into 400 radial spokes. Each section has an equal degree of angles in the angular coordinate. The 400 spokes are split up into eight groups and thus there are 50 spokes in each group. When the optical pickup unit circles around a ring of the label side of an optical disc, the optical pickup unit is pushed up by a triangular signal during the focusing on half of the 50 spokes, as shown in FIG. 2(a). The light summed signal is generated accordingly. As the optical pickup unit circles around another ring of the label side of the optical disc, the optical pickup unit is pushed down by another triangular signal during the focusing on half of the 50 spokes, as shown in FIG. 2(b). Another corresponding light summed signal is generated. As shown in FIG. 2(c), the light summed signals in FIG. 2(a) substrates the light summed signals in FIG. 2(b) group by group to generate difference values. The variation of the label side in the vertical direction can thus be determined via the difference values corresponding to each group of spokes. After the variation is obtained, the parameters of a corresponding control voltage are determined by adaptive control means and the optical pickup unit is controlled to move up and down to focus on the label side.
However, when the optical pickup unit moves from an inner circle to an outer circle to scribe a pattern on the label, the optical pickup unit may be seriously out of focus due to the variation in the vertical direction of the outer circle being much greater than the variation in the vertical direction of the inner circle. This problem can arise due to raising, buckling, or vibrating of the disc or the structure of the optical drive. When the problem occurs, even if the voltage of the triangular signal is adjusted, the optical pickup unit may remain out of focus. The light summed signal cannot reveal the difference precisely resulting in inaccurate parameters of the voltage control curve. A clear pattern on the label side therefore cannot be obtained and, in a worst situation, the pattern may not be able to be scribed. Hence the conventional label side focusing control method has problems that have to be solved.